1. Field of the Invention
This invention relates to an apparatus for applying a light beam to an optical recording medium having tracks provided thereon and effecting at least one of recording and reproduction of information while effecting tracking servo of the light beam.
2. Related Background Art
Various forms such as a disc-like form, a card-like form and a tape-like form are known as the forms of information recording medium for recording information thereon and reading out the information recorded thereon. Also, recently, attention has been paid to a method of applying a light beam condensed in the form of a spot to these mediums to thereby accomplish recording and reading-out of information, because of its capability of enhancing the recording density.
The manner of such recording will hereinafter be described by taking as an example a case where use is made of a card-like optical recording medium (hereinafter referred to as the optical card).
FIG. 1 of the accompanying drawings is a schematic plan view of the optical card, and FIG. 2 of the accompanying drawings is a fragmentary enlarged view thereof.
In FIG. 1, the letter C designates the optical card, the reference numeral 72 denotes information tracks, and the reference numeral 73 indicates the home position. Information is recorded as optically detectable record pit rows (information tracks) on the optical card C by causing a light beam modulated in accordance with recording information and reduced to a minute spot to scan the card C. At this time, to record or reproduce information accurately without causing any trouble such as crossing of the information tracks 72, it is necessary to control the applied position of the light beam in a direction perpendicular to the scanning direction (auto tracking, hereinafter referred to as AT). Also, to apply the minute spot stably in spite of the bending and mechanical error of the optical card, it is necessary to control the applied position of the light beam in a direction perpendicular to the optical card (auto focusing, hereinafter referred to as AF). As shown in FIG. 2, tracking tracks 75 (75.sub.1, 75.sub.2, ...) for effecting the aforementioned AT are provided between the information tracks 72 (72.sub.1, 72.sub.2, ...) of the optical card C.
A method of recording information on the optical card will now be described.
In FIG. 1, the light beam is at first positioned at the home position 73. The light beam is then moved in the direction of arrow D to seek an information track 72.sub.n to be recorded, and scans the information track 72.sub.n in the direction of arrow F, thereby accomplishing recording or reproduction of information. Track numbers are recorded on the information tracks 72, and by reproducing this information, the information track being currently recorded can be known. Also, recording of information is carried out in the fashion of a post script, i.e., in the order of the information tracks 72.sub.1, 72.sub.2, ..., 72.sub.n.
FIG. 3 of the accompanying drawings is a block diagram showing the construction of an information recording-reproducing apparatus for the optical card as described above.
In FIG. 3, a system controller 1 controls motors 2 and 3 and AT and AF servo circuits 4 and 5 to thereby control recording or reproduction of information on the optical card C.
The motor 2 reciprocally, moves the optical card C in the direction of double-headed arrow a, whereby a light beam is applied from an optical head 6 along the information tracks of the optical card C. The motor 3 is for moving the optical head 6 in a direction perpendicular to the information tracks on the optical card C.
The optical head 6 has a light source 6a comprising a semiconductor laser, a collimator lens 6b for collimating the light beam from the light source 6, a beam splitter 6c and an objective lens 6d, and applies a light beam for recording/reproduction onto the recording surface of the optical card C.
The optical head 6 also has a beam splitter 6e, condensing lenses 6f and 6g, and photoelectric converters 6h and 6i. The light beam reflected on the recording surface of the optical card C is received by the photoelectric converters 6h and 6i and converted into electrical signals thereby, and the electrical signals are supplied to the AT servo circuit 4 and the AF servo circuit 5, respectively.
The AT servo circuit 4 and the AF servo circuit send driving currents to the tracking coil 6j and focusing coil 6k of the optical head 6 to thereby move the objective lens 6d and effect AT and AF control.
The basic construction of such an information recording-reproducing apparatus is disclosed, for example, in U.S. Pat. No. 4,912,697.
Now, in the apparatus as described above, when a defect or defects are present on the recording medium or when an external shock is applied to the recording medium, there sometimes occurs abnormality to the tracking servo (that is, the phenomenon that AT deviates and the light beam does not correctly trace the information tracks). When such AT deviation occurs, the objective lens moves randomly, and this has led to the possibility of damaging the optical head. Also, particularly when AT deviation occurs during recording, the light spot strays on the medium, and this has led to the problem that information is superposedly written on the recorded information to cause the latter information to disappear.